1. Field of the Invention
The present invention relates to seed crystals, seed crystal holders, and a method for pulling a single crystal and, more particularly, to seed crystals, seed crystal holders, and a method for pulling a single crystal, which are used in growing a single crystal used as a semiconductor material.
2. Description of the Prior Art
There are various methods for growing a single crystal, one of which is a pulling method such as the Czochralski method (hereinafter, referred to as the CZ method). FIG. 1 is a diagrammatic sectional view showing the principal part of an apparatus for pulling a single crystal used for the conventional CZ method, and in the figure, reference numeral 31 represents a crucible.
The crucible 31 comprises a bottomed cylindrical quartz crucible 31a and a bottomed cylindrical graphite crucible 31b fitted on the outer side of the quartz crucible 31a. The crucible 31 is supported with a support shaft 39 which rotates in the direction shown by the arrow A in the figure at a prescribed speed. A heater 32 of a resistance heating type and a heat insulating mould 42 arranged around the heater 32 for promoting heat transmission to the crucible 31 are concentrically arranged around the crucible 31. The crucible 31 is charged with a melt 33 of a material for forming a crystal which is melted by the heater 32.
On the central axis of the crucible 31, a pulling axis 34 made of a pulling rod or wire is suspended, and at the lower end thereof, a seed crystal 35 is held by a seed crystal holder 10.
FIG. 2(a) is a partial sectional view diagrammatically showing a connecting construction of the seed crystal holder 10 in which the seed crystal 35 is inserted and the pulling axis 34, and FIG. 2(b) is a bottom face thereof.
The lower portion 35a of the seed crystal 35 is narrower than a shank portion 35b thereof, and a taper portion 35c thereof is formed between the lower portion 35a and the shank portion 35b.
The lower end portion 34A of the pulling axis 34 is in the form of a cylinder narrower than the body portion 34B thereof, and a groove 34a is formed on the side of the lower end portion 34A.
The seed crystal holder 10 is almost cylindrical, and a taper plane 10a is formed on the lower portion thereof. The seed crystal 35 is inserted in a hollow portion 11a in the lower portion of the seed crystal holder 10, while the lower end portion 34A of the pulling axis 34 is put in a hollow portion 11b in the upper portion of the seed crystal holder 10. A taper plane 11a.sub.-1 is formed in the middle of the hollow portion 11a. The wall surface of the hollow portion 11a is shaped according to the external form of the seed crystal 35 and the cone angle .theta..sub.1 of the taper plane 11a.sub.-1 and the cone angle .theta..sub.2 of the taper portion 35c of the seed crystal 35 are made to be the same. A groove 10b is formed on the upper side wall surface of the hollow portion 11b.
In order to make the seed crystal holder 10 having the above construction hold the seed crystal 35 and to connect the seed crystal holder 10 holding the seed crystal 35 to the pulling axis 34, the seed crystal 35 is inserted into the hollow portion 11a from the upper side of the seed crystal holder 10. By joining the taper portion 35c of the seed crystal 35 to the taper plane 11a.sub.-1 formed in the hollow portion 11a, the seed crystal 35 is held in the seed crystal holder 10 without dropping down out of the hollow portion 11a. The seed crystal holder 10 is connected to the pulling axis 34 by adjusting the hollow portion 11b of the seed crystal holder 10 to the lower end portion 34A of the pulling axis 34 and screwing the groove 10b formed in the seed crystal holder 10 to the groove 34a formed on the pulling axis 34.
Since the front portion of the seed crystal holder 10 is also exposed to a considerably high temperature when the seed crystal 35 is brought into contact with a melt 33 (about 1400.degree. C. or so), it is necessary to use a material excellent in heat resistance as a material for forming the seed crystal holder 10. And it is difficult to use a metal such as iron, since there is the possibility of the occurrence of heavy metal contamination. Therefore, in general, the seed crystal holder 10 is formed by a carbon material having a high heat resistance.
The above seed crystal holder 10 has a sufficient strength for pulling a single crystal 36 having a diameter of about 6 inches and a weight of 80 kg or so, which has been general hitherto. Recently, however, in order to make semiconductor devices more highly integrated at a lower cost and more efficiently, the wafer has been required to have a larger diameter. Now, for example, the production of a single crystal 36 having a diameter of about 12 inches and a weight of 300 kg or so is desired. In this case, when the seed crystal holder 10 is conventionally made of a carbon material, it cannot withstand the weight of the pulled single crystal 36 and breaks, resulting in the falling of the single crystal 36. The seed crystal holder 10 often breaks almost in parallel to the pulling direction as if to tear. When the seed crystal holder 10 comprises a metal, heavy metal contamination occurs or the strength thereof easily deteriorates by thermal fatigue in an early stage.
In order to solve the above problems, recently, a seed crystal holder made of graphite reinforced with carbon fibers has been used.
FIG. 3(a) is an oblique view diagrammatically showing a seed crystal holder made of graphite reinforced with carbon fiber cloth, and FIG. 3(b) is a sectional view thereof.
The seed crystal holder 45 comprises a large-diameter cylindrical portion 451 having a large-diameter cavity 451a, a small-diameter cylindrical portion 453 having a small-diameter cavity 453a, and the middle portion 452 which joins the large-diameter cylindrical portion 451 to the small-diameter cylindrical portion 453. The inner wall surface 452b of the middle portion 452 is an inclined plane. A groove 451b is formed on the inner wall surface of the large-diameter cylindrical portion 451, to which the front portion of a pulling axis 34 can be screwed. Since the shape of the large-diameter cylindrical portion 451 is not limited as long as it can be connected to the pulling axis 34, there are some cases wherein the groove 451b is not formed.
A seed crystal 35 inserted into the seed crystal holder 45 has an external form which fits into the form of the cavities 451a, 452a, and 453a formed in the seed crystal holder 45.
The seed crystal holder 45 is made of graphite reinforced with carbon fiber cloth, and is manufactured in the below-described manner. By impregnating carbon fiber cloth with a resin and laminating the same, a laminated body made of the resin including the carbon fiber cloth is formed. The laminated body is carbonized (graphitized) by heating it at a high temperature in an inert atmosphere. In this stage, the dipping of the laminated body into a pitch or the like and the step of carbonization are sometimes repeated plural times. The graphite reinforced with carbon fiber cloth is cut in the shape of the seed crystal holder 45, in which the cavities 451a, 452a, and 453a are formed. Then, by treatments such as grinding, the seed crystal holder 45 is manufactured.
The seed crystal holder 45 manufactured through the above steps has a large strength to a force in the vertical direction or in the parallel direction orthogonal to the laminating direction, but has a small tensile strength in the laminating direction, since the carbon fiber cloth for reinforcement is laminated in the parallel direction. Therefore, when a heavy single crystal is pulled, breaks sometimes occur between the laminae because the load of the heavy single crystal works on the inner wall surface 452b through the seed crystal 35.
The graphite reinforced with carbon fiber cloth in which carbon fiber cloth is laminated in the vertical direction is also used as a seed crystal holder, but since the laminating direction is vertical, it has a small strength to a force working in the vertical direction, so that breaks sometimes occur because of the load of the single crystal 36.
Ordinarily, unevenness of 150 .mu.m or more exists on the surface of the seed crystal 35. When a taper portion 35c has a weak place in strength caused by the unevenness, cracks proceed from the place, and the seed crystal 35 cannot withstand the weight of the single crystal 36 and sometimes breaks.
When the vertical section of the taper portion 35c comprises only straight lines, a boundary portion 35d of the taper portion 35c and a small-diameter lower portion 35a has a bending form, on which damage easily occurs if it is not treated carefully. When damage exists on the boundary portion 35d, cracks proceed from the boundary portion 35d in the pulling of the single crystal 36 and the seed crystal 35 sometimes breaks.
One of the important steps in the pulling of the single crystal 36 is the necking step.
Since the dislocation, which occurs in the seed crystal 35 in contact with a melt 33, generally tends to grow in the vertical direction to the growth interface of the single crystal 36, the shape of the growth interface (the front plane of a neck 36a) is made to be downward convex in the necking step, so as to exclude the dislocation.
In the necking step, the faster the pulling speed is made, the smaller the diameter of the neck 36a can be made and the more downward convex the shape of the growth interface becomes. As a result, the dislocation is inhibited from propagating and can be efficiently excluded.
In growing the heavy single crystal 36 having a weight of 300 kg or so, the diameter of the neck 36a needs to be about 6 mm or more in order to prevent the occurrence of troubles such as a fall of the single crystal 36 and to pull the single crystal 36 safely, which is calculated from the silicon strength (about 16 kgf/mm.sup.2). However, when the diameter of the neck 36a is 6 mm or more, the dislocation induced in dipping the seed crystal 35 into the melt 33 cannot be sufficiently excluded.